rotate around midpoint

Hi.

I feel like an idiot asking this question.

I'm trying to rotate a curve around its midpoint.

I thought setting the tvalue of the curve to .5 would give me the midpoint. (the curve is a straight line generated from 2 points)

Here's what I wrote: 

Dim pt As On3dPoint = crvA.PointAt(.5)

To get something that looks near the midpoint I have to set the value at .875

I'm guessing I'm not thinking about this correctly.